Aircraft heating apparatus with altitude responsive combustion air supply



Sept. 23, 1947. HOLTHOUSE 2,427,674-

AIRCRAFT HEATING APPARATUS WITH ALTITUDE RESPONSIVE COMBUSTION AIR SUPPLY Filed June 29, 1944 2 Sheets-Sheet l 96 TlMING DEVICE IN VEN TOR. HARRY B. HOLTHOUSE @354 awas m ATTORNEYS M 1947- H. B. HOLTHOUSE 2,427,674

AIRCRAFT HEATING APPARATUS WITH ALTITUDE RESPONSIVE COMBUSTION AIR SUPPLY Filed June 29, 1944 2 Sheets-Sheet 2 IN VEN TOR HARRY B; HOLTHOUSE BY M M )ZM oa/ ATTORNEYS Patented Sept. 23, 1947 UNITED STATES PATENT OFFICE AIRCRAFT HEATING APPARATUS WITH ALTITUDE RESPONSIVE COMBUSTION AIR- SUPPLY Application June 29, 1944, Serial No. 542,730

11 Claims. 1

The present invention relates to heating apparatus and more particularly to improvements in aircraft heaters of the internal combustion type adapted for uniformly reliable operation over a wide range of altitudes.

Many types of internal combustion heaters are available which will operate satisfactorily at the temperature and atmospheric pressure conditions prevailing at ground levels. In fact, certain of the commercially available heaters of this type are capable of operating with satisfaction at altitudes of approximately 15,000 feet. When this altitude is exceeded, however, the usual heater becomes unreliable and may become wholly inoperative due to the reduced oxygen content in the rarefied atmospheres at high altitudes and the cold temperatures encountered at such altitudes. As disclosed in applicants co-pending application Serial No. 435,845, filed March 23, 1942, now Patent No. 2,400,116, issued May 14, 1946, this problem is in part obviated by utilizing a motor, preferably of the series wound direct current type, having a drooping speed-load characteristic, to drive the combustion air moving means, whereby the motor speeds up to increase the volume of air delivered to the combustion chamber as the density of the air decreases with increasing altitude. This novel arrangement solves the problem entirely in operating an internal combustion heater at altitudes ranging up to approximately 20,000 feet. At still higher altitudes, however, other means must be relied upon for further'increasing the volume of air delivered to a heater combustion chamber if satisfactory combustion is to be obtained.

It is an object of the present invention, therefore, to provide improved aircraft heating apparatus in which completely reliable operation over a wide altitude range is obtained through the provision of an exceedingly simple arrangement for accentuating the described characteristic of the air moving motor to speed up as the density of the air moved thereby decreases with increasing altitude.

According to another object of the invention, a motor including differentially related series and shunt field windings is used to drive the combustion air moving means and facilities are provided for changing the current flow through at least one of the field windings or armature of the motor in the correct sense to accentuate the described speed characteristic of the motor as the altitude of heater operation is increased,

In accordance with a further and more specifi'c object of the invention, the desired increase 2 in speed of operation of the air moving motor with increasing altitude of heater operation is at least in part obtained by automatically changing the length of the air gap separating the motor armature from the adjacent pole pieces as the altitude of heater operation is changed.

More generally stated, it is an object of the present invention to provide improved aircraft heating apparatus of the internal combustion type which is completely reliable in operation over a wide range of altitudes.

The invention both aS to its organization and method of operation, together with further objections and advantages thereof will best be understood by reference to the following specification taken in connection with the accompanying drawings, in which:

Fig. 1 is a diagrammatic view of improved aircraft heating apparatus characterized by the features of the present invention;

Fig. 2 is a longitudinal sectional view of the apparatus shown in Fig. 1 illustrating the meohani 1 construction and arrangement thereof;

Fig. 3 is a side sectional view of the fuel conditioning unit forming a part of the apparatus shown in Figs. 1 and 2;

Fig. 4 is an end sectional view of the combustion chamber provided in the apparatus; and

Fig. 5 is a diagrammatical illustration of the manner in which the combustion and circulating air delivery conduit as provided in the apparatus of Figs. 1 and 2 is arranged with respect to one of the propellers of an aircraft in which the apparatus is installed.

Referring now to the drawing and more particularly to Fig. 1 thereof, the present improved heating apparatus is diagrammatically illustrated as comprising means defining a combustion chamber 17 which is surrounded by an air circulating chamber 29 having an outlet opening 3'! through which air heated by contact with the walls defining the combustion chamber I! may be discharged to the space to be heated. The combustion chamber is provided with an inlet for admitting a fuel and air mixture from a fuel conditioning unit 22 which is positioned within an air pressure chamber l6, and with a discharge orifice 2| through which the products of combustion are discharged after burning within the chamber I'l. Combustion air for supporting combustion within the chamber I! is first delivered to the chamber I6 under pressure by means of a fan 33 and is then admitted to the combustion chamber from the pressure chamber in the manper specifically described below. Air is circu- 3 lated through the chamber 29 and discharged through the opening 3'! into the space to be.

heated by means of a second fan 32 to which air is admitted through a conduit 61. These two fans, which are preferably of the well known sirocco type, are commonly mounted: for rotation upon the rotor shaft 33 of a compound wound direct current motor 34. In brief, this motor comprises a field structure having tapered pole faces 34b spaced from a tapered armature 34a so that an air gap 34c of generally'conical configuration is provided therebetween. It is equipped with a series field winding IOI which is adapted to be energized in-series withthe windings of the armature 34a through the'customary commutator 34c and brushes 34d from a direct current source 92. Thisseries energizing circuit also includes a fuel and air heating element 57 which is included in the fuel conditioning..unit.22, andone or .more of three resistors I I3a, I I32) and I I 30 which shunt the series winding ml and have the function of variably controlling the voltage. applied. to the armature.

The motor'34 further comprises a small shunt Winding I02 which is differentially-related to the series winding. -IOI and is arranged for energization from the source 92 overa circuit which may includea variable number or none of the resistors II5a,.II5b and IISc. This differential winding is such size that the vfluxproduced thereby is incapable of predominating over the-flux produced by the series winding II'II over the operating speed-range of the motor and, yet'is of sufiicient size to appreciably weaken the series field when fully energized.

For the purpose of changing thespeed-load characteristicof the motor 34 in a manner such that the. air moving'fans .are rotated. at increasingly higher speeds with a decrease in .the'density in the air moved thereby, a speed "responsive device9 is provided which; is driven. by the 'rotor shaft'33. This device has'the functions of controlling the length .of the air gap. 34c separating the armature periphery from the. pole faces of the field pole pieces, controlling the energization of the differential shunt winding I02, and controlling the voltage applied'to the armature 34a- It is schematically illustrated as comprising fl balls I supported by'resilient elements l'0'I between two rings I03 and I04; the first of which is rigidly mounted upon the shaft 33- and the second ofwhich is movable axiallyof this shaft. More specifically, the arrangement is such that when the shaft 33 is rotated at a predetermined speed, the fly balls I06 act to move the ring I04 into engagement with-a stationary bearing ring I carried by the motor housing in any conventional manner.

'in'Fig. 1 of the drawings; thereby to increase the length of the air gap 34c. 'is noted that the construction of the motor is In this regard it such that substantial movement of the shaft 33 and the parts carried thereby between two predetermined axial positions is permitted. Like- 'wise,the fans 32 and-38 must be freely axially movable within theirhousings to permit the described axial displacement of the shaft 33.

With the described arrangement, movement of the shaft 33 axially to the left may be utilized to "control in steps the value of the resistance shunting the-series field IOI; and also to control the 4 energization of the differential field I02. To this end, the end 3311 of the shaft is arranged to engage an L-shaped rocker arm I08 pivoted at I09 and engaging contact closing members III and II2 respectively associated with the contacts IIS and- II4. A tension pring H0 is utilized to tension one leg of the arm I08 into engagement with the end 33a of the shaft 33 for the purpose of maintaining the shaft 33 and the parts carried thereby in the position at which the shortest air gap 3 Ie-prevails. between the periphery of the armature'taand the pole faces 34b. In maintaining theshaft 33 in this position the arm I08 alsoholds the contact actuating elements I I I and II 2.-in settings such that the respective associated contacts Ilfia to H611, inclusive, and II4a to I I4c,'inclusive, are open. The electrical equipment for the heating apparatus further comprises a solenoid actuated pump 4! having a Winding 4Ia arranged for intermittent energization through a pair of contacts'42 which are adapted to'be'periodically opened and closed through the action'of a timing device 06. This device maybe of any conventional'form. A manually operable two-step switch 88, comprising a wiper arm33 and a pair of associated contacts 99 and. I00, is provided for selectively controlling the energization of the winding IIa and the various described windings of the motor 34.

Referring now more specifically to the construction and arrangement of the. heater the combustion' chamber I7 is shown in Figs. 1 and 4 as being divided longitudinally into four axially extending but connected passages I 'Ia,IId .by a partition member I8 .of substantially X-shape. The combustion chamber inlet I9.and outlet 2| are formed in the bottom portion I4 of thedishshaped member 20 in communication with the passages I'Ia and I'Id, respectively. The air and fuel'mixing unit 22lis located within the inlet I9 and extendsiwithin the air supply chamberIB. "Theoutlet 2I is provided with a tail pipe 23 extended through the air supply chamber IS and outwardly from the heater at thehousing end 2 1.

The outer wall or body portion of the combustion chamber I7 is. provided withangularly spaced axially extending fins 26. These fins have a sleeve '28 positioned about the outer ends thereof to form an annular air circulating passage 29 surrounding the combustion chamber I'I. .Air to be heated. is admitted into the passageZS through an inlet 3|.connecting the passage with the mechanical compartment I2 and is in, part circulated through the passage 29 by the .fan 32 located within the compartment I2. The compartment I2.and air passage..29..are separated from the air. pressure chamber. I3 by a ealing or partition member 36 extended transversely of the housing I0. From Fig. 2-of the drawings, it will :be seen that the air supply chamber I6 isdcflned by the member 20, the partition member36, and theend 24 of the housing I0. Air circulated by the fan'32 is'thus confined to travel within the compartment I2 and passage 29 and is. discharged from the passage through an outlet; 37 which is connected to a space to be heated.

The airsupply chamber I8 receives air from the fan 38 located thereinand mounted on the motor shaft 33 which is journaled in the partition plate 36. An inlet 39 for the fan 38 is provided in the housing end 24. Fuel for the pump il is supplied thereto from a suitable source (not shown) through a pipe 53 and is delivered through a pipe 44 to a. fuel injection nozzle 45 forming a 'part'of the air and fuel mixing means 22. The

.pump 4|, motor 34 and fan 32 are thus all located within the mechanical compartment [2, which is provided with an inlet 41 for supplying air to the fan 32 for delivery to the air passage 29.

Briefly considered, the air and fuel mixing means 22 includes a casing member 38 which is closed at one end and opens into the passage Ila at the opposite end 49 thereof. A mixing chamber 5| is located at the closed end of the casing 48, and is separated from an equalizing chamber 52 by a perforated plate member 53. The equalizing chamber 52 in turn is both defined and separated from the combustion chamber passage lla by a perforated heat insulating plate 54 spaced inwardly from the end 49 of the casing 48. Extended substantially axially through the casing 48 and supported in the partition plates 53 and 54 to project outwardly from the closed end of the casing 48 is a combination electric heating and igniting unit 56 which includes a resistance coil 51 supported in a spaced relation within a metal tube 53.

In the operation of the air and fuel mixing means 22, the fuel delivered to the nozzle 45 by the pump Al is directed into the mixing chamber 5!, the fuel nozzle being located within the air supply chamber 16 and mounted directly on the casing 48 to communicate with the mixing chamber. A portion of the air for mixing with the fuel enters the nozzle 46 from the air chamber l6 through ports 59 in the fuel nozzle and travels with this fuel into the mixing chamber 5|. Additional air from the air chamber I 5 is admitted directly into the mixing chamber 5i through apertures Bl formed in the casing 48 about the fuel injection nozzle 66. The fuel within the mixing chamber 5| is heated to at least a fuel vaporizing temperature by the combination unit 56 to facilitate its thorough mixing with the air. The casing 48, partition plate 53 and tube 58 are constructed of a good heat conducting material so as to readily transfer the heat radiated by the coil 5'! to the air and fuel mixture. From the mixing chamber, the vaporous air and fuel mixture .passes through the perforated plate 53 into I the equalizing chamber 52 which, in cooperation with the heat insulating plate 54, acts to reduce the turbulence in the mixture and to disperse the mixture substantially uniformly over the entire cross section of the casing 38. This combustible mixture then passes through the apertured plate 54 and across the open end 62 of the tube 58 into the effective igniting zone of the coil 5'5, thereby to ignite the combustible mixture.

Admission of fuel to the pump 4| is controlled by a valve unit 86 which is connected in the supply line 43 and includes the valve and switch actuating element 81. This element is extended through the body of the unit 86, is provided with an operating handle at one end, and is operatively connected at its opposite end with the control switch 88. The arrangement is such that upon movement of the actuating element 81 to fully close the valve unit 86, the switch 88 is operated to its open circuit position.

For the purpose of utilizing the air pressure developed during flight of an aircraft 8 in which the apparatus is installed, to assist in delivering combustion air to the combustion chamber H and circulating air to the circulating chamber 29, and more specifically, to create air pressures at the inlet openings 39 and 41 which assist in the transmission of air to the chamber fl and through the chamber 29, conduit means are provided which include a common portion 65 having an opening 9! extending to the outside of the craft and facing in the direction of flight. In other-words, the opening 9| faces against the direction of air flow about the surfaces of the craft in which the apparatus is installed. More specifically, and as best shown in Fig. 5 of the drawings, this opening faces directly into the slip stream, indicated at 1, generated by the propeller 6 of one of the engines of the aircraft 8. It has been found that with this arrangement, the air pressure developed within the entrance part of the conduit E5 during operation of the aircraft remains reasonably constant regardless of the speed of operation of the craft and regardless of the altitude at which the craft is operated. Otherwise stated, the air pressure within this portion of the conduit 65 is positively generated 'by the propeller 6, such that fluctuations therein are largely eliminated even though the craft is operated over a wide range of speeds and altitudes.

From the above explanation, it will be understood that to initiate operation of the heating apparatus, the valve and switch operating element 81 is first actuated to a position wherein the contact wiper 98 engages the contact I00. With the arm 93 in this position, a circuit is completed for energizing the heating element 51 in series with the series field I0! and the armature 34a of the motor 34. Accordingly, the heating element 5'! is energized to vaporize any fuel collected within the fuel conditioning unit 22, and the fan 38 is started in operation to scavenge the combustion chamber I! of any combustion products accumulated therein, all before operation of the fuel pump 4| is initiated to deliver fuel to the conditioning unit. After a short time interval, the switching and valve actuating element 81 may be thrown to the run position wherein fuel is admitted to the inlet side of the pump 4! through the feed line 43. Also in response to this operation the contact wipin arm 98 and the contact 99 are engaged to energize the operating magnet lla of the fuel pump 4| through the intermittently closed contacts 42 which are constantly actuated by the timing device 96. Accordingly, the fuel feed system is rendered operative to deliver fuel to the fuel conditioning unit 22. In this unit the fuel is vaporized and mixed with the combustion air traversing the air inlet opening 39 and is discharged into the combustion chamber H for ignition, all in a manner which will be fully apparent from the preceding explanation. In this regard it will be understood that combustion air is delivered to the fuel conditioning unit 22 for mixture with the vaporized fuel over a path which includes the common conduit 65, the branch conduit 53, the chamber IG and the orifices through the housing of the fuel conditioning unit and associated nozzle. Combustion air is also delivered to the combustion chamber I! through the passages 63 which surround the fuel conditioning unit 22 in the end wall Id. Thus, it will be apparent that the pressure which forces combustion air into the combustion chamber H is in part derived from operation of the fan 33 and in part by the pressure developed within the conduits 65, E6 and 57 as a result of the air driven into the entrance end of the conduit 65 by the propeller 5. More specifically, as the propeller 6 of the craft in which the equipment is installed is operated, a substantial and reason ably constant air pressure is built up within the conduit 65 at the inlet sides of the fans 32 and 38.

- So long as theaircraft in which the illustrated heating equipment is installed is operated at low altitudes the speed of operation of the motor 34 remains substantially constant. Accordingly, combustion air is delivered to the chamber I'I.at a substantially uniform rate entirely adequate for satisfactory combustion therewithin. In this regard, it is noted that the fans 32 and 38 are of relatively large size such that at the air densities which prevail at ground level and 'low altitudes, the motor 34 is slightly overloaded. Accordingly, at such altitudes the speed of opera tion of the motor is held down due to the load imposed thereon by the two fans 32 and 38, although this speed is entirely adequate to insure that the requisite amount of combustion air will be supplied to the combustion chamber I'I. Further, the volume of air circulated through the chamber 29 at such levels is entirely adequate to carry off the major portion of the heat developed by the combustion of the fuel mixture within the chamber I'I.

As previously indicated, the motor 34 has all of the characteristics of a series wound direct current motor, such that it will inherently seek a speed at which it is fully loaded. In other words, the motor seeks to retain a constant load thereupon and to this end will change its speed of operation in response to tendencies of the load to increase or decrease. Accordingly, when the heating apparatus and more properly the aircraft in which this apparatus is installed is operated at altitudes ranging from 15,000 feet upward, the motor 34 tends to speed up as a result of the tendency of the load on the fans 32 and 38 to decrease, due to the decreasing density of the air at such altitudes. As the motor 34 speeds up, the volume of air delivered to the combustion chamber I7 is increased to correspondingly increase the amount of oxygen available for supporting combustion within this chamber. Concurrently, the volume of air passed through the air circulating chamber 29 is increased to compensate for the decrease in the heat-absorbing properties of the air as the density thereof decreases.

From the above explanation it will be understood that as the altitude of aircraft operation is increased above 15,000 feet, the speed of operation ,of the motor 34 progressively increases. Incident to tlns increasing speed of rotation of the shaft 33 the sleeve I04 is moved to the right under the influence of the centrifugally actuated fly balls I05 until a point is reached at which this sleeve engages the stationary bearing ring I05. During the continuing increase in the speed of rotation of the shaft 33 which occurs incident to a further increase in the altitude of operation of the craft, this shaft and the parts carried thereby are moved axially to the left as viewed in Fig. 1 of the drawings to correspondingly increase the length of the air gap 34c separating the periphery of the armature 34a from the pole faces 34b. As a result, the field in which the armature 34 is rotating is correspondingly decreased to produce a corresponding increase in the motor speed. In other words, increasing the air gap 34a in a progressive manner has the effect of progressively accentuating the characteristic of the motor to speed-up with decreasing density of the air moved by the two fans 32 and 38. This characteristic is further accentuated through the action of the arm I08 to progressively close the contacts II4a, H412 and H40, in the sequence named, for the purpose of progressively decreasing the resistance of the path shunting the series winding IOI thereby correspondingly weakening the field produced by this series-winding. Concurrently with the weakening of the field in the described manner, the voltage applied to the armature 34a is obviously increased to still further increase the speed of the motor for any given load. A still further accentuation in the series unwind characteristic of the motor 34 is obtained through movement of the arm I08 to first energize the differential winding I02 and to then progressively increase the energization of this winding. Thus following engagement of the sleeve or collar I04 with the bearing ring I05 and during continued movement of the shaft 33 to the left, the arm I I38 is pivoted in a clockwise direction about its pivot point I09 against the action of the tension spring I I0. As this pivotal movement of the arm I08 proceeds, the contacts 411, H41) and H40 are successively closed in the order named to successively exclude the resistors II3a, I I3b and H30 from the path shunting the series winding I 0!, until at the maximum speed of operation of the motor only the residual resistor MM is connected in shunt with this winding. Concurrently, with the described step by step decrease in the current traversing the series winding WI and accompanying increase in voltage applied to the armature 34a, the arm I08 acts through the contact closing member I II successively to close the contacts II6a, IIIib, II6c and HM. Incident to closure of the contacts IIBa a circuit is completed by way of the brush 98, the contact I00 and the three series connected resistors H50, H51) and 5a. for energizing the differential shunt winding I02 from the current source 92. Energization of this winding serves further to reduce the effective field in which the armature 34a is operating, thereby further to accentuate the described speed-load characteristic of the motor 34. As the sequential closure of the contacts IIB proceeds in the manner just described, the three resistors II5a, II5b and H50 are successively excluded from the energizing circuit for the winding I02 thereby progressively to increase the energizing of this windingand thus progressively decrease effective field invwhich the armature 34a is rotating.

By a proper correlation of the three related sets of described facilities for accentuating the series unwind characteristic of the motor 34 this motor may be designed to operate at substantially any desired speed for any desired altitude of aircraft operation. In other words, the described facilities for increasing the length of the air gap 346, decreasing the energization of the series winding I02, increasing the voltage applied to the armature 34a, and energizing and progressively increasing the energization of the differential winding I 02, all in response to increasing altitude of operation, permit the speed-load characteristic of the motor 34 to be so predetermined that an extremely wide variation of motor speed is obtained in response to the change in air load imposed upon the fans 32 and 38 which occurs as the altitude of operation of the apparatus is changed. Although not specifically explained above, it will be understood that as the altitude of operation of the apparatus decreases to increase the load imposed upon the fans 32- and 38, by virtueofthe increasing density of the air moved thereby, the spring IIO acts in conjunction withthejfiy ball actuated sleeve I04 progressively..tolIdecrease the length of the air gap 34e, increase the energization of the series winding IUI and decrease the energization of the differential shunt winding I02. The manner in which this is accomplished will be fully apparent from the above explanation. It will also be understood that the altitude at which the described facilities for accentuating the series unwind characteristic start to operate is determined by the position of the bearing ring I relative to the movable ring I04 in a direction axially of the shaft 33. By making the ring I05 adjustable along the shaft 33, the altitude at which these facilities start to function may be varied as desired.

While one embodiment of the invention has been disclosed, it will be understood that various modifications may be made therein which are within the true spirit and scope of the invention as defined in the appended claims.

I claim:

1. In aircraft heating apparatus of the internal combustion type, means defining a combustion chamber, air moving means for delivering combustion air to said combustion chamber, a

motor for driving said air moving means, said motor having the characteristic of increasing its speed with decreasing density of the air moved by said air moving means, whereby said air moving means tends to speed up as the density of the air decreases with increasing altitude, and means responsive to increasing speed of said motor for increasing the rate at which the speed of said motor increases with decreasing density of the air moved by said air moving means.

2. In aircraft heating apparatus of the internal combustion type, means definin a combustion chamber, air moving means for delivering combustion air to said combustion chamber, a motor for driving said air moving means, said motor having the characteristic of increasing its speed with decreasing density of the air moved by said air moving means, whereby said air moving means tends to speed up as the density of the air decreases with increasing altitude, and means responsive to increasing altitude of operation of said apparatus for progressively increasing in steps the rate at which the speed of said motor increases with decreasing density of the air moved by said air moving means.

3. In aircraft heating apparatus of the internal combustion type, means defining a combustion chamber, air moving means for delivering combustion air to said combustion chamber, a motor for driving said air moving means, said motor having the characteristic of increasing its speed with decreasing density of the air moved by said air moving means, whereby said air moving means tends to speed up as the density of the air decreases with increasing altitude, and means including a speed responsive device driven by said motor for increasing the rate at which the speed of said motor increases with decreasing density of the air moved by said air moving means when the density of the air moved by said air moving means falls below a predetermined value.

4. In an aircraft heater of the internal combustion type, means defining a combustion chamber, air moving means for delivering combustion air to said combustion chamber, a motor for driving said air moving means, said motor including differentially related series and shunt field windings and having the characteristic of increasing its speed with decreasing density of the air moved by said air moving means, whereby said air moving means tends to speed up to increase the volume of air delivered to said combustion chamber as the density of the air decreases with increasing altitude, and means responsive to increasing altitude of operation of said heater for changing the current flow through at least one of said windings in the correct sense to accentuate the described speed characteristic of said motor.

5. In an aircraft heater of the internal combustion type, means defining a combustion chamber, air moving means for delivering combustion air to said combustion chamber, a motor for driving said air moving means, said motor including differentially related series and shunt field windings and having the characteristic of increasing its speed with decreasing density of the air moved by said air moving means, whereby said air moving means tends to speed up to increase the Volume of air delivered to said combustion chamber as the density of the air decreases with increasing altitude, a speed responsive device driven by said motor, and means controlled by said speed responsive device for changing the current flow through at least one of said windings in the correct sense to accentuate the described speed characteristic of said motor as the altitude of operation of the heater is increased from a predetermined Value.

6. In an aircraft heater of the internal combustion type, means defining a combustion chamber, air moving means for delivering combustion air to said combustion chamber, a motor for driving said air moving means, said motor including an armature and having the. characteristic of increasing its speed with decreasing density of the air moved by said air moving means, whereby said air moving tends to speed up to increase the volume of air delivered to said combustion chamber as the density of the air decreases with increasing altitude, a speed responsive device driven by said motor, and means controlled by said speed responsive device for increasing the current flow through said armature as the altitude of operation of said heater increases, thereby to accentuate the described speed characteristic of said motor.

'7. In an aircraft heater of the internal combustion type, means defining a combustion chamber, air moving means for delivering combustion air to said combustion chamber, a motor for driving said air moving means, said motor including stator pole pieces and an armature separated by an air gap, and means responsive to increasing altitude of operation of said heater for increasing the length of said air gap to increase the speed of rotation of said armature, thereby to increase the volume of air delivered to said combustion chamber with increasing altitude of operation of said heater.

8. In an aircraft heater of the internal combustion type, means defining a combustion chamber, air moving means for delivering combustion air to said combustion chamber, a motor for driving said air moving means, said motor including stator pole pieces and an armature separated by an air gap, and speed responsive means driven by said motor for increasing the length of said air gap in response to increasing altitude of operation of said heater, thereby to increase the speed of rotation of said armature and thus increase the volume of air delivered to said combustion chamber.

9. In an aircraft heater of the internal combustion type, means defining a combustion chamber, air moving means for delivering combustion air to said combustion chamber, a motor for driving said air moving means, said motor including stator pole piecesand an armature con- 'structed 'tobe separated by a conical air gap and having the characteristic of increasing its speed with decreasing density of the air moved by said air moving means, whereby said air moving-means tends to speed up to increase the volume of air delivered to'said combustion chamber as the density of the air decreases with increasing altitude, and means responsive to increasing altitude of operation of said heater for moving said armature and stator pole pieces relative to each other in the correct direction to increase the length of said air gap, thereby to accentuate the described speed characteristic of said motor and thus further increase the volume of air delivered to said combustion chamber with increasing altitude of operation of said heater.

In an aircraft heater of the internal combustion type, means defining a combustion chamber, air moving means for delivering combustion air to said combustion chamber, a motor for driving said air moving means, said motor including stator pole pieces and an armature constructed to be separated by a conical air gap and having the characteristic of increasing its speed with decreasing density of the air moved by said air moving means; whereby said air moving means tends to speed up to increase the volume of air delivered to said combustion chamber as the density of the air decreases with increasing altitude, and speed responsive means driven by said motor for moving said armature and stator pole pieces relative to each other in the correct sense to increase the length of said air gap as the speed of said motor increases,

thereby'to accentuate the described speed characteristic of sa'id'motor and further thus increase the volume of air'delivered to said combustion chamber with increasing altitude of operation of said heater.

'11. In an aircraft heater of the internal combustion type,'means defining a combustion chamber, air moving means for delivering combustion air to said chamberja motor for driving said air moving means, said motor having the characteristic of speeding up as the density of the air moved by said air moving means decreases with increasing altitude, thereby to increase the volume of air delivered to said combustion chamber as the altitude of operation of said heater is increased, a speed responsive device driven by said motor, and means responsive to operation of said device for automatically accentuating said characteristic of said motor as the altitude of operation of said heater is increased, thereby to reduce variations in the amount of combustion supporting oxygen delivered to said combustion chamber over a wide range of altitudes.

HARRY B. HOLTHOUSE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name c Date 1,562,663 Strong Nov. 24, 1925 2,314,089 Hess et a1. Mar. 16, 1943 2,353,201 Talbot July 11, 1944 2,348,113 Davis May 2, 1944 2,364,214 Hess et al. Dec. 5, 1944 

